A "Combo" is done by priming it and then detonating it with a different character.
If a "Combo" is primed on an enemy you will see a red symbol above its head.
Normally Supports/Tanks are Primers and Trickster/Damage roles are Detonators with Seven as an exeption.

Detonators:

Ace: "Rocket Fist"
Jimmy: "Multi-Grenade" Stage 2 and 3

Primers:

Pinkyy: "Leap"
Raff: "Keytar Blast" and "Deadly Drop" Skill in "BAD GIRL" Skilltree
Seven: "Maelstorm"

If a "Combo" is made you will notice a big "Combo" above the Target.
The Most effective detonator is Ace in my oppinion, because he can use his Fist more then Jimmy´s nade.

Hope This was a bit helpfull to clear things up.



Note:This is also explained in the game but some ppl might have